All modern microphones utilize a membrane or a solid plate as a diaphragm to absorb acoustical energy from sound pressure waves. That energy is then converted to electrical impulses or digital signals by a variety of means, depending on the microphone design. The impulses or signals are then stored or transmitted for immediate or later reproduction by headphones or loudspeakers.
The diaphragm or flat plate introduces distortions, non-linear effects, and attenuation into the signal. This is the inevitable consequence of the physical nature of the device. While sound waves travel in only one direction from the source (reflected energy from other surfaces complicates the situation), the diaphragm or plate must travel in two directions, forward and back, in order to maintain its position in the microphone housing. This undesirable bi-directional operation inherent in traditional microphone design is remedied with the present invention.
There are a number of U.S. Patents that disclose methods for detecting sound waves in air by using lasers and other optical methods, attempting to detect the change in density of the airflow caused by sound pressure waves, or indirectly by measuring the deflection of a surface responding to the pressure waves. The prior art includes the following patents: U.S. Pat. No. 6,301,034, U.S. Pat. No. 6,147,787, U.S. Pat. No. 5,785,403, U.S. Pat. No. 4,479,265, U.S. Pat. No. 4,412,105, U.S. Pat. No. 6,598,853, U.S. Pat. No. 6,483,619, U.S. Pat. No. 6,154,551, U.S. Pat. No. 6,055,080, U.S. Pat. No. 6,014,239, U.S. Pat. No. 5,262,884, and U.S. Pat. No. 4,166,932.
The measurement of smoke density in a flue is common within industrial facilities to monitor pollutants and process state. Smoke density in exhaust pipes is also commonly measured to evaluate the performance of diesel engines.
Current microphone technology has two fundamental and irreducible problems: (1) the diaphragm or plate that detects sound pressure waves has a finite mass; and (2) as a consequence, the diaphragm or plate takes a finite amount of time to respond to changes in sound wave pressure.
These two problems are a source of non-linear response and loss of audio information by the microphone. These non-linearities and losses are difficult to quantify for the simple reason that the detection methods used to study these problems contain the same flawed transducers they are attempting to measure.
For the sake of illustrating the nature of the non-linearities and losses of a conventional microphone, consider the case where a 2,000 Hz steady-state audio tone is suddenly changed to a 4,000 Hz tone at half the volume. For this change to be accurately recorded, the output signal must change to its new state within 0.00025 seconds. Within that period of time, the diaphragm, membrane or plate and any attached metal coil or magnet inside the microphone capsule must increase its linear speed by a factor of two, and at the same time reduce its linear excursion (travel) by half. In fact, there are no physical transducer systems that can accomplish this; all systems with mass necessarily have some hysteresis effects.
Depending on the mass of the moving elements in the microphone, the actual transition from old to new output signal will be on the order of ten times the period required to avoid distortion and signal loss. Consequently, for the duration of time it takes for the microphone to respond to the new signal and have no remnants of the previous signal, the new 4,000 Hz signal is corrupted in both frequency and amplitude by the microphone's physical “memory” of the discontinued 2,000 Hz signal. In real-life situations, where the input sound waves are constantly changing, this problem is exacerbated. Listeners perceive this problem as the part of the difference between recorded audio and live audio. The goal of the present invention is to reduce that perceived difference as much as possible.